Low NOx emission diffusion flame combustor for gas turbines

ABSTRACT

A diffusion flame combustor for gas turbines comprising a fuel injector unit ( 4 ) mounted at one end of a flame tube ( 6 ) delimits a combustion chamber ( 6   a ) and has its other end connected to a duct ( 11 ) that conveys the combustion gases to the turbine. The flame tube is coaxially mounted inside a tubular container ( 2 ) that communicates with the discharge casing ( 5 ) of an air compressor and, together with the tube, defines a space ( 30 ) for the combustion air. The injector unit ( 4 ) comprises an injector head ( 9 ) facing into the combustion chamber ( 6   a ) and provided with at least one fuel gas injection nozzle ( 29 ), which comprises an air-gas premixing duct ( 28 ) into which the fuel gas is introduced, the duct having an inlet section that communicates with the air space ( 30 ) and being provided with a radial-fin air swirler ( 31 ) arranged upstream of the point at which the fuel gas enters said premixing duct ( 28 ). The partial premixing of the combustion gas with the combustion air assists in reducing the NOx emissions and the production of unburnt materials.

DESCRIPTION

[0001] 1. Field of the Invention

[0002] The present invention relates to a low NOx (nitrogen oxides) emission diffusion flame combustor for gas turbines. The combustor in accordance with the invention is particularly suitable for retrofitting to industrial-type gas turbines with combustors of the can-annular type, such as for example the MS-5000 gas turbines marketed by General Electric-Power System (GE-PS).

[0003] 2. Description of the Prior Art

[0004] As is known, a diffusion flame combustor for gas turbines comprises a fuel injector unit mounted at one end of a flame tube that delimits the combustion chamber and has its other end connected to a duct that conveys the combustion gas to the turbine blades. The flame tube is coaxially mounted within a tubular container that communicates with the combustion air compressor discharge casing and, together with the flame tube, defines a space for the combustion air. The injector unit is mounted so as to have its head inside the combustion chamber and is provided—in the two-fuel version, for example—with an axial atomizer nozzle for the liquid fuel and, all around it, an annular injector nozzle or a crown of nozzles for the injection of the gaseous fuel. The air arriving from the air space is conveyed into the combustion chamber independently of the injector unit. Holes appropriately distributed along the flame tube are provided for the input of the primary, secondary and dilution air.

[0005] This type of simple-cycle gas turbine engine, today used throughout the electric power industry for peak generation, can have emission levels too high with respect to the site limits and therefore can be subjected to power limitations or operation prohibition in many sites. In particular, there are worldwide thousands of gas turbine engines operating for peaking load services; gas and oil firing NOx emissions, at full load engine conditions, are approximately 0.17 and 0.3 Kg/Mcal respectively. NOx emission levels of this type are above those prescribed in many countries where average NOx emission limits are below 0.13 Kg/Mcal; in the next few years a further reduction of the emission limits to 0.065 Kg/Mcal or even lower is foreseen. Accordingly, it has customarily been necessary to average the NOx emissions from the gas turbine engines with other sources operating well below the system-wide emission limits in order to achieve compliance.

[0006] In a gas turbine combustor NOx emissions are essentially generated by two mechanisms:

[0007] 1) Primary mechanism: by fixation of atmospheric nitrogen in the flame (thermal NOx);

[0008] 2) Secondary mechanism: by conversion of nitrogen chemically bound in the fuel (chemical NOx), as in some lower quality heavy fuel oils, process gases and some coal gases from gasifiers with hot gas clean up.

[0009] According to the Zeldovich mechanism, the rate at which thermal NOx is formed increases exponentially with the flame temperature and linearly with the time the combustion gas remains at the flame temperature. Consequently, the peak flame temperature and the dwell time are the principal variables that control NOx formation and the resulting emission levels. Furthermore, the rate at which nitrogen oxides are formed diminishes rapidly when the flame becomes poor in fuel and as the peak temperature diminishes. The introduction of small quantities of diluents into the primary combustion zone therefore has the effect of cutting down the rate of thermal NOx formation.

[0010] Consequently, to maintain in operation old GT engines, when more stringent emission limits are required, the possible choices must be considered among the following options:

[0011] a) installation of water or steam injection systems on gas turbine engines to achieve 40 to 50% NOx reduction, with reference to actual emission values. This requires extra equipment and increases operation costs due to loss of engine efficiency caused by high value of water/fuel ratios;

[0012] b) installation of sophisticated and very expensive dry low-NOx combustion systems that also require considerable modifications to the engine control equipment, if they are based on multistage premixed combustion processes;

[0013] c) retrofitting combustion systems with minor modifications so as to achieve both low NOx emission levels and low impact on the engines.

[0014] With a view to rendering the emissions of machines fired with natural gas compatible with the emission limits today in force, GE-PS recently developed low NOx emission versions of the combustor for their MS-5000 machines. Without making use of DLN technology, the solution proposed by GE-PS achieves the reduction of the NOx emissions by varying the distribution of the air within the combustor by means of a modification of the sizes and the pattern of the holes in the flame tube (thereby stepping up the primary air flow) in such a way as to assure lean mixing ratios in the primary combustion zone at maximum load.

[0015] This solution, which calls for nothing other than modifications of the flame tube, has the advantage of being easy to realize and implement in the machine in the course of ordinary maintenance. Nevertheless, it may lead to an increase in the production of unburnt matter, because the temperature peaks in the primary combustion zone will become attenuated and thus reduce the reaction rate in the diffusion flame. Since the geometry of the combustion system remains constant, it is not possible for the dwell time to be stepped up.

OBJECTS AND SUMMARY OF THE INVENTION

[0016] The object of the present invention is to provide a combustor for diffusion flame gas turbines that, thanks to an optimal distribution of the air and fuel in the primary combustion zone, will allow a substantial lowering of the NOx emissions as compared with the combustors that are conventionally used for the same application.

[0017] A particular purpose of the present invention is to provide a combustor of the aforementioned type suitable for being retrofitted to a diffusion flame gas turbine in the power range of 15-24 Mwe, capable of giving rise to a modest impact on the machine and being applied to it, maintaining its original encumbrance, without having to modify the control system or the fuel supply of the original machine.

[0018] These aims are attained by the combustors in accordance with the present invention in which the gas injection nozzle comprises a duct in which the air and the fuel gas become premixed, the entrance section of the duct communicating with said air space and being provided with an air swirler arranged just upstream of the point at which the gas enters the premixing duct.

[0019] This solution increases the air in the primary combustion zone and realizes a partially premixed air-fuel stream to be exposed on the flame front in such a way as to limit the temperature peaks that are among the principal factors promoting the formation of thermal NOx. In this way, moreover, the increase in the production of unburnt matter that would naturally occur in the diffusion flame as a result of the attenuation of the primary flame temperature is limited due to the effect of the premixing.

BRIEF DESCRIPTION OF THE DRAWINGS

[0020] These and other characteristics of the combustor for diffusion flame gas turbines in accordance with the present invention will be brought out more clearly by the following description of a particular embodiment thereof, which is to be considered solely as an example and not limitative in any way, said description making reference to the attached drawings of which:

[0021]FIG. 1 shows a combustor assembly in accordance with the present invention;

[0022]FIG. 2 shows a cross section through the injector unit of the combustor in accordance with the present invention;

[0023]FIG. 3 shows details of a different embodiment of the premixing duct;

[0024]FIG. 4 shows a front elevation of the injector unit as seen from inside the combustion chamber;

[0025]FIG. 5 shows a graph that compares the emissions of the original combustor (STD) with those of the new combustor (LED) in accordance with the present invention;

[0026]FIG. 6 shown a diagram illustrating the reductions of the NOx emissions in various operating conditions that can be obtained with the combustor in accordance with the present invention and compares them with the emissions due to a diffusion flame combustor of the known type.

DETAILED DESCRIPTION OF THE INVENTION

[0027] Referring to FIG. 1, the reference number 1 indicates a generic combustor unit of a gas turbine comprising an outer tubular container 2 that is closed at one end by a cover 3 to the centre of which there is fixed an injector unit 4. The other end of tubular container 2 communicates with the discharge box 5 of a combustion air compressor, not shown. A flame tube 6 is arranged coaxially with tubular container 2 and delimits combustion chamber 6 a, which is closed at one end by a calibrated plate 7 that is uniformly perforated and at its centre is provided with a ferrule 8 to engage head 9 of injector unit 4.

[0028] Referring now to FIG. 2, from ferrule 8 there extends a wall 10, in the form of a truncated cone, connected to a sleeve 10 a coaxial with flame tube 6 and constituting the capping of combustion chamber 6 a. As shown in FIG. 4, the surface of wall 10 is provided with uniformly distributed raised cross-cuts to create passages for the combustion air. The other end of flame tube 6 engages with a conveyor duct 11 that feeds the gas turbine, not shown. Along flame tube 6 delimiting the combustion chamber there are provided primary holes 12, secondary holes 13 and dilution holes 14 for the combustion air, which flows from discharge box 5 into an air space 30 between flame tube 6 and tubular container 2. A flame conduit 15 is also provided to put into communication the combustion chambers of adjacent combustor units.

[0029] As shown in greater detail in FIGS. 2 and 4, injector unit 4 comprises a body 16 provided with a nozzle 17 for the liquid fuel, a nozzle 18 for the atomization air and a nozzle 19 for the gas. Injector unit 4 also comprises a flange 20 by means of which it is connected to cover 3 of outer tubular container 2. Inside body 16 there are provided a central duct 21 for the liquid fuel and, around said duct 21, an inner bundle of ducts 22 for the atomization air and an outer bundle of ducts 23 for the gaseous fuel. The head 9 of the injector unit comprises a centrally arranged atomizer nozzle 24 for the fuel and coaxially connected to central duct 21. A chamber 25, where ducts 22 collect the atomization air, is provided outside nozzle 24, the air being injected in a conventional manner by means of radial canals in the zone immediately downstream of nozzle 24 on the end plane at right angles to the axis of the combustor.

[0030] Gas ducts 23, on the other hand, lead into a more outward annular chamber 26 communicating through respective passages 27 with an annular premixing chamber 29 situated within gas injector 29. In greater detail, as can be seen from FIG. 2, atomizer nozzle 24 is coaxially contained within a tubular body 33 that is integral with body 16 of injector unit 4 and, together with it, delimits chamber 25 for the atomization air. Gas injector 29, in its turn, is made up of an inner flange element 34 fixed to the outside of tubular body 33 and an outer flange element fixed to ferrule 8, which together define annular gas duct 28.

[0031] At the inlet end of annular duct 28, which communicates with the space 30 for the combustion air, there is provided—upstream of gas passages 27—a radial swirler 31 that causes the combustion air to become turbulent and thus facilitates its mixing with the gas in premixing duct 28.

[0032] Annular premixing duct 28 comprises a radial part (the swirler 31 being situated at the entry thereto) and an axial part—these parts being joined by a curved portion devoid of sharp edges—and has a cross section that gradually becomes smaller as the outlet end in combustion chamber 6 a is approached. More particularly, the section at the output end of duct 28 is reduced to between 20% and 28% of the section at the input end. This configuration makes it possible to obtain a uniform premixing that prevents the formation of ring vortices that could cause potentially harmful oscillation phenomena within the machinery.

[0033] In the illustrated embodiment the radial swirler has twelve fins with a swirl angle of 30° and handles a quantity of air equal to 9-12% of the total combustion air. Twelve gas passages 27, each having a diameter of 4.5 mm, are provided and spaced evenly around a circumference situated in the vicinity of the outlet end of swirler 31.

[0034] The gas injection passages may also be more numerous (up to 40), just as the swirler fins may vary in both number and inclination, though remaining of the radial type.

[0035] As shown in FIG. 3, moreover, gas injection passages 27 may be distributed on two or more circumferences in such a way as to subdivide the air input into a series of successive phases. This may prove advantageous or even necessary to avoid pre-ignition or flash-back problems in the case of machines operating at a high compression ratio (temperature of combustion air greater than 330° C.) or when the walls of duct 28 become very hot.

[0036] The use of a part of the combustion air as air to be premixed with the gas in the injector implies a corresponding reduction of the dilution air and therefore a consequent re-sizing of dilution holes 14 on flame tube 6, which have to be re-designed in both section and number to minimize the “traversing quality factor” at the entry to the turbine blades.

[0037] In the combustor in accordance with the invention the air is used to control the temperature in the primary combustion zone and the configuration of the combustor assures the formation of a primary combustion zone that, on average, is lean and homogeneously premixed to obtain very low NOx levels. The equivalence factor in the primary combustion zone is Φ=0.7-0.85 rather than about Φ=1 as in traditional combustion. Flame stabilization is obtained by appropriately designing the angle of the radial blades so as to create a recirculation zone downstream of the injector that will permit ignition at a low load, followed by a gradual increase of both speed and load in regular running conditions. The optimal values of the force and extent of the recirculation zone are the consequence of optimal relations between the momentum of the air drawn in through the radial swirler and flowing in the narrowing duct and the momentum of the air passing through the primary holes.

[0038] The combustor in accordance with the invention achieves the following results:

[0039] reliable and uniform distribution of air and fuel at the entry of the combustor using a strong swirling action to reduce NOx formation;

[0040] optimized distribution of the combustion air for greater operational flexibility with maximum stability of the flame and minimal CO production as compared with the injection of water or steam;

[0041] a stable primary recirculation zone useful for fast ignition, maintaining the existing ignition systems;

[0042] integration with water injection systems for further lowering the NOx and CO emissions to comply with future emission limits (0.06-0.09 kg/Mcal).

[0043] More particularly, as brought out by the graphs reproduced as FIGS. 5 and 6, in dry operating conditions it is possible to obtain 50% of the reduction of the NOx emissions with only very limited modifications of the machine and then to further reduce the NOx emissions to 65% by adding a water injection system to the machine.

[0044] Naturally, the optimization of the combustor in accordance with the present invention was based on operation with natural gas. However, it can also be advantageously employed with liquid fuels, though in that case, as can be seen from FIG. 6, the obtainable NOx reduction levels will not be as good as in the case of gas operation.

[0045] Obviously, when the invention is applied to larger can-annular combustors, the size of the combustor can be appropriately varied in scale in accordance with the principle of proportionality and for powers greater than those indicated herein it is always possible to obtain further appreciable reductions of the NOx emissions by means of water injection.

[0046] Variations and/or modifications could be brought to the diffusion flame combustor for gas turbines in accordance with the present invention without departing from the scope of the invention as defined by the claims hereinbelow. 

1. A diffusion flame combustor for gas turbines comprising a fuel injector unit mounted at one end of a flame tube that delimits a combustion chamber and has its other end connected to a duct that conveys the combustion gases to the turbine, said flame tube being coaxially mounted inside a tubular container that communicates with the discharge casing of an air compressor and, together with said tube, defines a space for said combustion air, said injector unit comprising an injector head facing into said combustion chamber and being provided with at least one fuel gas injection nozzle, wherein the fuel gas injection nozzle comprises an air-gas premixing duct into which the fuel gas is introduced, said duct having an inlet section that communicates with said air space and being provided with a radial-fin air swirler arranged upstream of the point at which the fuel gas enters said premixing duct.
 2. A combustor in accordance with claim 1, wherein said fuel gas injection nozzle has an annular form and said injector head comprises a liquid fuel nozzle centrally positioned with respect to said gas injection nozzle.
 3. A combustor in accordance with claim 1 or 2, wherein there is provided a plurality of passages along said premixing duct for the introduction of the fuel gas.
 4. A combustor in accordance with claim 3, wherein said passages for the fuel gas are arranged in one or more axially concentric rows.
 5. A combustor in accordance with claim 4, wherein said passages for the fuel gas are arranged in two axially concentric rows in order to subdivide the introduction of the gas into the combustion air into two successive phases.
 6. A combustor in accordance with claim 1, wherein the outlet section of the premixing duct is reduced to 20-28% of the inlet section of the duct.
 7. A combustor in accordance with claim 1, wherein said premixing duct is formed by a pair of coaxial and mutually spaced flanges, i.e. an internal and an external flange, integral with the body of the injector unit, said passages for the fuel gas being arranged along the internal flange.
 8. A combustor in accordance with claim 1, wherein said premixing duct consists of a radial part, at the inlet end of which there is arranged said swirler, and an axial part, said two parts being joined by a curved portion devoid of sharp edges. 